High pressure water has been used in the past for cleaning an undesirable layer of material from a base or substrate of desired material. For example, in recent years, the descaling of hot steel billets in rolling mills has become a necessity because it improves the quality of the final steel products, and such higher quality has become necessary for these products to be competitive in international markets. For this purpose, many steel rolling mills in the United States now discharge relatively large volumes of water against the hot steel billets in order to use thermal shock to remove scale from billet surfaces before the billets are shaped.
In one such application, a rolling mill has used two large high-pressure pumps and an array of eight fan jet nozzles mounted to provide a spray ring for covering all sides of a hot billet moving through the ring. Such fan jet nozzles provide a coreless spray having a wide fan-like shape which diverges at an angle in the range of about 15.degree. to about 40.degree. as measured from side to side in the plane of the "fan". Water was delivered to the spray ring at a pressure of about 1800 psi, and was discharged at the rate of about 100 gallons per minute. In addition to such high water usage at relatively low pressures, fan jet nozzles have another disadvantage in that their diverging spray causes the water to lose its energy rapidly, so that by the time the water hits the surface to be cleaned, it does not have sufficient energy to knock off the scale by impact. Fan jet nozzles therefore rely on thermal shock principles to remove the scale rather than the impact energy of high-velocity water streams.
Fan jet water sprays also have been used to clean dried resin-impregnated fibrous material from the perforated steel drums and calendar rolls of dryers used in processing continuous webs of fleece-like materials, such as those dryers manufactured by the Fleissner Company of Germany and used in the manufacture of carpeting. In one such application, the water pressure was raised to 36,000 psi in an effort to get sufficient water impact to remove the dried resinous coatings from the perforated steel drums. Although this water cleaning effort was successful, it required very large volumes of water and was extremely slow. For example, 12-18 hours were required to clean the two drums of a single dryer at a cost of about $400 per hour. The significant variation in hours required was due to differences in the depths of the coatings allowed to accumulate on the drums before they were cleaned.
Instead of water cleaning with fan jets, mechanical cleaning methods also have been used in the past to remove an outer layer of undesirable material from a base or substrate of desired material. One such mechanical cleaning method has been used to clean deposits of electrolytic baths from spent anodes used in the metal refining industry. For example, one plant for refining aluminum has used rotary blast wheels employing centrifugal force to throw steel shot against the bath coating on the carbon body of the anodes after they were dried. By abrading the dry bath coating, the steel shot created a large volume of dust which had to be collected by means of a dust collector and bagged in 1,000 pound bags for disposal in a landfill. Thus, the aluminum refining plant generated 16 tons of dust per week, which cost about $300 per week merely to haul to a landfill. Moreover, 16 dust bags at a cost of about $320, and about 1 ton of steel shot at a cost of about $400, were expended each week. Such abrasive cleaning systems also are labor intensive, and involve high maintenance costs. The yearly maintenance on the shot blasting machines at the aluminum refining plant was over $20,000. Workers in the area also had to be protected against the dust such that such abrasive cleaning techniques are not environmentally friendly.
Mechanical abrading methods have also been used for cleaning Fleissner dryers of the type mentioned above. In one plant, a sandblasting and vacuuming machine was used to traverse the drums, but proved to be too slow and after several cleanings, the drums had to be removed and either resurfaced or replaced at a cost of about $35,000 per drum. Sandblasting also involves a dusty environment in which the workmen must wear breathing masks. In addition, sand and other abrasive particles can get into the moving parts of machinery and cause these parts to wear more quickly.
The cleaning of Fleissner dryers also has been carried out in at least one plant by hand by putting several workmen with breathing masks inside each dryer and having them use electric wire brushes and abrasive pads. This method took four men three days to clean the surfaces of the two drums of a single dryer, and the holes through these surfaces could only be partially cleaned. Therefore, when the dryer was put back in operation, air would not pass as efficiently through the partially clogged holes, and this significantly slowed down the rate of production in this plant.
For many cleaning situations, it has been considered impractical in the past to use pressurized air to remove a moderately adhered layer or coating of undesirable material, such as bath deposits, from a substrate of desired material, such as a carbon anode.